* C pointer types always have allowzero set to true but they omit the
word allowzero when printed.
* Implement coercion from C pointers to other pointers.
* Implement in-memory coercion for slices and pointer-like optionals.
* Make slicing a C pointer drop the allowzero bit.
* Value representation for pointer-like optionals is now allowed to use
pointer tag values in addition to the `opt_payload` tag.
Companion commit to 61a53a587558ff1fe1b0ec98bb424022885edccf.
This commit also moves over a bunch of behavior test cases to the
passing-for-stage2 section.
Conflicts:
* doc/langref.html.in
* lib/std/enums.zig
* lib/std/fmt.zig
* lib/std/hash/auto_hash.zig
* lib/std/math.zig
* lib/std/mem.zig
* lib/std/meta.zig
* test/behavior/alignof.zig
* test/behavior/bitcast.zig
* test/behavior/bugs/1421.zig
* test/behavior/cast.zig
* test/behavior/ptrcast.zig
* test/behavior/type_info.zig
* test/behavior/vector.zig
Master branch added `try` to a bunch of testing function calls, and some
lines also had changed how to refer to the native architecture and other
`@import("builtin")` stuff.
